On March 4th, SpaceX tried to land a Dragon rocket on a barge 200 miles from land in the Atlantic Ocean. The attempt failed. Just as SpaceX’s head Elon Musk said it would. But, says Musk, the next attempt—the fifth—just may succeed. SpaceX wants to make that next attempt on April 8th with a flight launching at 4:43 EDT from Cape Canaveral “if the barge is ready.”

Why is Musk trying so hard to land on a moving target? Why is he spending so much time and money to perfect an impossible trick, landing on a floating platform at sea? Landing on a platform that might be jostled by the currents or heaved up and down by ocean waves? After all, two companies—Musk’s SpaceX and Jeff Bezos’s Blue Origin–have now managed to do something far easier, to ease down on the launch site from which their journey began. Why not stick with what you’ve already proven you can do? Why not land your rocket on land?

The answer? Land on a barge and you can increase the size of the payload that you deliver to orbit. Land on what SpaceX calls an Autonomous Spaceport Drone Ship (ASDS), and the rest is a matter of fuel economics.

The first stage of a Falcon rocket launching a cargo to orbit achieves a speed of 4,970 miles an hour. That high-speed flight takes the rocket a long way from its launch pad. It takes the rocket over the ocean, between 160 and 200 miles out. If the rocket were forced to go back to its launch pad, it would have to burn about 30% of its fuel to make what SpaceX calls “a screeching U-turn” and power the long flight back home. It would need a lot of fuel to do the equivalent of reversing a cannonball in flight and making it return to the muzzle from which it came That 30% of fuel could be used to launch heavier cargos and bigger satellites. Or satellites headed for the lofty heights required for geostationary orbit. So there’s a challenge. How do you save that 30% of your fuel and make it available for the delivery of more payload?

Instead of traveling back to your launch pad, you make your launch pad travel to you. And that’s what barges do. Autonomous Spaceport Drone Ships position themselves where the rocket will be when it lands. Where it will be, that is, if it falls from the sky with just a twitch of rocket guidance, if it takes the simplest, most fuel-efficient path back to earth.

Why is Elon Musk trying so hard to land on a moving target? Why has he tried four times to land on a barge? Why has he made these landing attempts even when he knew they would fail? To perfect the art. And to do more than just increase the size of payloads and the ease of reaching geostationary orbit. To ready his company for its massive upcoming space ship scheduled for rollout toward the end of this year, the Falcon Heavy.